This invention relates to the field of latent image development systems and more particularly to a novel method for developing a colored image on a substrate by application of a color developing agent, comprising diphenylcarbazone or its derivatives, over a latent image containing a metal salt with which the color developing agent is reactive.
Latent image systems are used for a variety of purposes. In testing and instruction, the printing of correct test or workbook answers in a color developable latent image facilitates grading or self-instruction. Such color developable images also find application in various children's amusement devices such as coloring books, games and the like. A rather extensive art has developed in which various color development systems are used to render latent images visible.
Thus, for example, Fraik U.S. Pat. No. 4,111,702 describes a system in which a latent image containing dithiooxamide or a derivative thereof is developed by the use of a wax crayon which contains a coreactant metal salt and a trialkyl or triaryl phosphate as an image forming reaction accelerator. Salts of nickel, copper, cadmium and iron are disclosed as reactive with the dithiooxamide derivative to produce color.
Panken U.S. Pat. No. 3,979,550 describes pre-printed latent image spirit duplicating masters in which the latent image contains a color former such as dithiooxamide, N,N'bis(2-hydroxyethyl) dithiooxamide, N, N'didodecyl dithiooxamide, N,N'dimethyl dithiooxamide, or crystal violet. Development of the latent image is provided by use of a pen or crayon containing a nickel salt reactive with the latent image color former.
Buerkley et al. U.S. Pat. No. 4,850,649 describes a system in which a latent image containing a polyhydroxyaromatic complexing agent for iron, such as a derivative of gallic acid, pyrogallol or catechol, is developed using a wax crayon containing an iron salt.
Skinner U.S. Pat. No. 3,363,336 relates to a device for teaching writing skills, including a visible control image and a second image that is at least partially invisible and related in geometrical configuration to the control image. A wide variety of color development systems are disclosed including pH indicators, starch/iodine, clay/crystal violet lactone, lead acetate/xylenol orange, propyl gallate/iron salt, polyvinylpyrrolidone/dimethyl glyoxime/nickel chloride, and copper chloride/potassium succinate.
Lange et al. U.S. Pat. No. 4,111,462 and U.S. Pat. No. 4,205,865, Yarian U.S. Pat. No. 4,334,015, and Tomlinson U.S. Pat. No. 4,322,466 all relate to pressure releasable encapsulated color activating systems. The Lange patents disclose a latent image containing a transition metal salt of an aliphatic carboxylic acid which is complexed with dithiooxamide. Yarian utilizes an aryl substituted hydrazone corresponding to the formula ##STR1## where Ar.sub.1 and Ar.sub.2 are independently selected aromatic substituents and the coreactant is a transition metal. Tomlinson teaches the chealating of Fe.sup.+3 or Cr.sup.+2 with EDTA, tannic acid, gallic acid, glyoximes or benzophenones. Other systems disclosed by Tomlinson use ink containing 2[bis(hydroxyphenyl)methyl] benzoic acid, 3,3-bis-[4-hydroxy2-methyl-5(1-methylethyl)]1 3H isobenzofurone or 4-nitrophenol, developed with an alkaline solution such as KOH or K.sub.2 CO.sub.3.
Adachi et al. U.S. Pat. No. 3,843,384, Kubo Japanese Pat. No. 1974:33670, and Adachi et al. Japanese Pat. No. 1976:03647 disclose thermosensitive copying materials in which two components that are reactive to form a color product are dispersed in a binder. Upon contact with a heated pen or impingement of infrared radiation, the binder softens and allows contact between the components so that the color reaction occurs in the heated area. Among the components disclosed for use in carrying out the color reaction are various metal soaps as electron donors and diphenylcarbazone or another carbazone derivative as an electron acceptor.